1 Prof. Jones: So if you're Merlin, then this chair must really have belonged to King Arthur.
2 Merlin: In a manner of speaking, yes. It's the Siege Perilous.
3 Prof. Jones: The Siege Perilous! The magical seat cursed by Merlin to slay whomsoever dares sit in it!
3 Merlin: Precisely…
4 Prof. Jones: Well it was cold enough to give me a jolly case of the piles, but I don't think it was that bad.

I remember my mother telling me when I was young not to sit on cold surfaces like metal or concrete because it would give you piles. I had no idea what piles were at the time, but it sounded bad enough that I was careful about ever sitting on anything cold.

It turned out that piles is another name for haemorrhoids, and that this whole "sitting on cold surfaces" thing is a myth, as a quick Internet search will now tell you. Doing the research just now I also learnt that there are similar myths about sitting on cold surfaces causing: bladder infections, kidney infections, and even female sterility caused by damage to the ovaries. The things you learn!